1. Field of the Invention
This invention relates to a process for recovering kraft chemicals from spent cooking liquors in the paper pulping process. In particular, the invention relates to a novel way of processing the liquors as a slurry rather than as a homogeneous solution.
2. The Prior Art
In a typical papermaking process, pulp is prepared from wood by either chemical or mechanical pulping processes. In the more common chemical process, wood chips are loaded into a pressure vessel known as a digester which is charged with a chemical reagent. Heat is supplied to the digester to "cook" the wood chips to remove undesirable substances such as lignin from the wood and to liberate the desirable cellulosic fiber.
A widely used chemical pulping process is the "kraft" pulping process, which uses an alkaline chemical reagent referred to as "white liquor" to act upon the lignin in the material. Typically, the white liquor is an alkaline solution of sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S), provided by an aqueous solution typically containing between about 80-90 grams/liter of NaOH and about 20-25 grams/liter Na.sub.2 S with minor amounts of sodium carbonate, sulfate and thiosulfate. Depending upon the wood species used and the desired end product to be manufactured, white liquor is added to the wood chips in sufficient quantity to provide a total charge of alkali of 15-20% Na.sub.2 O based on the dried weight of the wood.
Typically, the temperature of the wood/liquor mixture is maintained at about 165.degree.-170.degree. C. for a total reaction time of about 2-3 hours. When digestion is complete, the pressure in the digester is released and the resulting wood pulp is separated from the spent liquor, called "black liquor" in a series of washing operations.
The pulping chemicals are recovered from the black liquor by a process that is commonly referred to as the recovery process. The black liquor, which contains organic constituents and sodium, is concentrated and then burned in a process referred to as smelting to reduce the organic constituents to CO.sub.2 and H.sub.2 O. The ash from the smelting operation containing sodium carbonate, or soda ash, is mixed with water to form a weak solution known as "green liquor". The solid particles in the green liquor, the "dregs" are then removed from the green liquor. The green liquor is subsequently causticized through the addition of lime to regenerate white liquor. This entire process can be time consuming and expensive.
It has been known that low grade fuels such as industrial waste may be more efficiently converted to energy if the fuel is provided in a slurry and heated at high pressure until the chemically bound oxygen is converted to CO.sub.2 gas. Such a process is described in U.S. Pat. No. 5,485,728 to Dickenson.
Thermal treatment thus appears to have beneficial properties in the processing of waste materials. Its application to the chemical recovery in paper pulping processes has not been done prior to the present invention but would appear to offer beneficial results.